It has been reported that with low molecular weight organic EL (hereinafter abbreviated as OLED) devices, provision of a copper phthalocyanine (CuPC) layer as a hole injection layer enables the drive voltage to be lowered and initial characteristics, such as an luminous efficiency, to be improved and also a life characteristic to be improved (Non-Patent Document 1: Applied Physics Letters, United States of America, 1996, Vol. 69, p. 2160-2162).
On the other hand, it has also been reported that with organic EL (hereinafter abbreviated as PLED) devices using polymer light-emitting materials, when thin films made of polyaniline materials (Patent Document 1: JP-A 3-273087, Non-Patent Document 2: Nature, Britain, 1992, Vol. 357, p. 477-479) and polythiophene materials (Non-Patent Document 3: Applied Physics Letters, United States of America, 1998, Vol. 72, p. 2660-2662) are used as a hole-transporting layer, similar results as in the OLED device are obtained.
In recent years, it has been reported that charge-transporting varnishes have been found, which are made of homogeneous solutions obtained by completely dissolving low molecular weight oligoaniline materials or oligothiophene materials of high solubility in organic solvents. When the hole injection layer obtained from the varnish is inserted into an organic EL device, a flattening effect of an underlying substrate and excellent EL device characteristics are obtained (Patent Document 2: JP-A 2002-151272, Patent Document 3: WO 2005/043962 Pamphlet.
The low molecular weight oligomer compound per se is low in viscosity, for which where an ordinary organic solvent is used, a process margin in film-forming operations becomes narrow, thus making it difficult to form a film having high uniformity when using a variety of coating processes such as spin coating, ink jet coating, spray coating and the like and various baking conditions. However, the use of different types of solvents to be added enables the viscosity, boiling point and vapor pressure to be controlled and thus, it becomes possible to obtain a film surface having high uniformity in correspondence with different types of coating processes (Patent Document 4: WO 2004/043117 Pamphlet, Patent Document 5: WO 2005/107335 Pamphlet).
The reason why no solid precipitation takes place irrespective of the addition of a variety of solvents as set out above and the solution is kept uniform is due to the high solubility and non-agglutinative nature of the low molecular weight oligomer compound. Accordingly, the solubility characteristics of coating-type charge-transporting materials are very important.
In recent years, in view of the problems involved in the solvent resistance of ink jet coating apparatus and the solvent resistance characteristic of structures on substrates such as an insulating film, a partition wall and the like, there has been demanded coating with a charge-transporting varnish, especially, using a low-polarity solvent.                Non-Patent Document 1: Applied Physics Letters, USA, 1996, Vol. 69, p. 2160-2162        Non-Patent Document 2: Nature, UK, 1992, Vol. 357, p. 477-479        Non-Patent Document 3: Applied Physics Letters, USA, 1998, Vol. 72, p. 2660-2662        Patent Document 1: JP-A 3-273087        Patent Document 2: JP-A 2002-151272        Patent Document 3: WO 2005/043962 Pamphlet        Patent Document 4: WO 2004/043117 Pamphlet        Patent Document 5: WO 2005/107335 Pamphlet        